The invention relates to a method for dewatering sludge on a wire, which is cleaned by washing nozzles and subsequently enters the sludge inflow area, where a flocculant is added to a sludge and the sludge is then at least partially dewatered, during which the rheological properties of the sludge on the wire are scanned optically and the amount of flocculant to be added is set according to the free surface area of the wire in a control zone. It also relates to a device for dewatering sludge, with a flocculant feed device and a wire, which is cleaned by washing nozzles before entering the inflow area, arranged after the flocculant feed device, where a device, especially a camera, is provided in order to scan the rheological properties of the sludge optically and is connected via a control system to the flocculant feed device in order to control the flocculant dosage added.
Methods for dewatering sludges are known from the state of the art, where sludges such as sewage or fibrous sludge are dewatered with flocculant added. Here, the flocculant is added to the sludge in a first stage in order to cause the sludge to flocculate, after which the sludge is applied to a wire, on a gravity table for example, so that the liquid in the sludge drains off through the wire, while dry sludge flocks remain on the wire. This causes the sludge to be dried or dewatered, respectively.
In methods of this kind, flocculant dosage is an important parameter. On the one hand, a desired dry content can only be achieved with the right dosage because too much or too little flocculant both have a negative effect on the efficiency of the dewatering or screening process. On the other hand, the flocculant itself entails costs, so adding too much flocculant also makes the method more expensive.
Normally, the amount of flocculant added to the sludge is set manually by the operating personnel, who assess the flocculation status of the sludge visually. However, this makes considerable demands on the operating personnel, both in terms of their ability to judge the correct flocculation status and of their perseverance and vigilance. Thus, an operator often cannot tell whether dewatering performance can be improved by increasing or by reducing the flocculant dosage because both too much and too little flocculant will result in poorer dewatering efficiency. Correct flocculant dosage is difficult to achieve in the result of state-of-the-art methods, which means that the processes are not run at the optimum operating point in most cases, with the result that the methods only achieve low dewatering performance or high flocculant consumption.
Automated methods are known, e.g. according to U.S. Pat. No. 5,380,440, where a visual signal from the surface of a wet solids layer on a belt is recorded and compared with a pre-set value for moisture content. It is possible here to achieve optimum flocculant consumption for dewatering to a certain moisture content at a specific point on the belt, but not to the optimum or minimum moisture content. In addition, a system is known from US 2007/0090060 that considers a control zone located after the dewatering ploughs for improving dewatering. Due to fouling of the belt and the uneven flow behaviour of the individual sludges, a high flocculant dosage is needed here primarily in order to see the desired streaks as the basis for measurement. As a result, the measurement is severely distorted and inaccurate and cannot result in a minimum flocculant dosage.